Interpretive Summary: Control of temperate tree fruit pests is mainly achieved using a series of chemical insecticide applications, and this form of control presents hazards to the environment, workers and consumers. An available alternative to chemical insecticide use is mating disruption (permeation of sex pheromones which prevents males of a pest species from locating females). Researchers at the USDA-ARS Yakima Agricultural Research Laboratory (Wapato, WA) and the University of California, Davis (Davis, CA) used the navel orangeworm as a model moth species to identify pheromone receptors that are responsive to sex pheromones and a sex pheromone analog. A sex pheromone analog was discovered that is more potent in activation of a pheromone receptor than the sex pheromone. Pheromone receptors activate a signaling pathway that is thought to be important for males locating females during mating periods, and are also targets for the development of new attractants or repellents to prevent males from finding females during reproduction. Successful characterization of this receptor will directly apply to efforts leading to a greater understanding of how male pheromone receptors work in the moth’s ability to find females and result in the development novel forms of control of temperate tree fruit pests.

Technical Abstract:
The insect’s olfactory system is so selective that male moths, for example, can discriminate female-produced sex pheromones from compounds with minimal structural modifications. Yet, there is an exception for this “lock-and-key” tight selectivity. Formate analogs can be used as replacement for less chemically stable, long-chain aldehyde pheromones, because male moths respond physiologically and behaviorally to these parapheromones. However, it remained hitherto unknown how formate analogs interact with aldehyde-sensitive odorant receptors (ORs). We identified olfactory receptor neurons (ORNs) housed in trichoid sensilla on the antennae of male navel orangeworm that responded equally to the main constituent of the sex pheromone, (11Z,13Z)-hexadecadienal (Z11Z13-16Ald), and its formate analog, (9Z,11Z)-tetradecen-1-yl formate (Z9Z11-14OFor). We cloned an odorant receptor co-receptor (Orco) and aldehyde-sensitive ORs from the navel orangeworm, one of which (AtraOR1) was expressed specifically in male antennae. AtraOR1'AtraOrco-expressing oocytes responded mainly to Z11Z13-16Ald, with moderate sensitivity to another component of the sex pheromone, (11Z,13Z)-hexadecadien-1-ol. Surprisingly, this receptor was more sensitive to the related formate than to the natural sex pheromone. By contrast, a male antennae-specific pheromone-binding protein, AtraPBP1, showed stronger affinity for the natural pheromone, but attempts to test with the Xenopus oocyte system whether they attenuate the formate response were unsuccessful, possibly because of high levels of PBP and lack of conformational change to mimic the moth’s olfactory system. A pheromone receptor from Heliothis virescens, HR13 (=HvirOR13) showed a similar profile, with stronger responses elicited by a formate analog than to the natural sex pheromone, (11Z)-hexadecenal.